The invention relates to a method of manufacturing a press felt, the method comprising forming a laminated base fabric having at least two superimposed, independent, woven layers, each layer having its own warp yarns and weft yarns; assembling the base fabric from one or more base fabric modules into one closed loop; providing for assembly jointing edges at the opposing edges of the base fabric; and attaching at least one batt fibre layer to the base fabric after the assembly.
The invention further relates to a press felt, which comprises a woven base fabric and at least one batt fibre layer, and in which the base fabric comprises at least two superimposed, independent layers, each layer having its own warp yarns and weft yarns, the base fabric further comprising at least one base fabric module, which base fabric module is provided with necessary jointing edges, of which jointing edges one or more base fabric modules are joined into a closed loop base fabric.
The invention also relates to a base fabric, which comprises at least two superimposed, independent layers, each layer having its own warp yarns and weft yarns, and which base fabric comprises at least one base fabric module which is provided with necessary jointing edges, of which jointing edges one or more base fabric modules are joined into a closed loop base fabric.
The aim of a press section in a paper machine is to drain a web as efficiently as possible without causing any deterioration in the quality of the web, however. From the viewpoint of energy consumption, it is most advantageous to remove as much water as possible already at the press section, whereby there is less need to dry the paper web at subsequent sections of the paper machine. Depending on the press structure, the press section of the paper machine employs a press felt on one or both sides of the web to be dried, into which press felt the water in the web is absorbed. The purpose of the press felt is to transport the water away after pressing without allowing it to migrate back into the web. In pressing, the paper web is conveyed on the felt to a gap between two rolls, i.e. so-called nip. Typically, there are one to four nips in succession, and in each nip water is pressed out of the web into the felt. Consequently, the felt should be such that the water can well be absorbed into the felt pressed in the roll nip. The press felts comprise a base fabric that provides the felt with a necessary space for water, for instance. To make the felt surface smooth, batt fibre is needled onto a base fabric surface facing the paper web. The batt fibre thus prevents markings from being produced on the web to be drained. In addition, by means of the batt fibre the water retention capacity of the felt can be adjusted to a desired level such that water is prevented from migrating back from the felt into the web, i.e. so-called rewetting is prevented. Further, the press felt should endure pressing, in order not be permanently compressed, and thereafter, easily blocked.
Even though non-woven arrays of yarns and corresponding structures have also been developed, the base fabric of the press felt is still typically manufactured by weaving, because better stability is achieved by a woven structure. Further, weaving allows better formation of a base fabric to suit each purpose, because there are more structural alternatives. Several alternative ways to weave the base fabric are known. The base fabric can be woven on a weaving machine directly into an endless loop. In that case, the weft yarns, transversal while being woven, are in the machine direction during use on the paper machine, whereby the width of the weaving machine limits the length of the base fabric to be manufactured. Using this technique, it is difficult to manufacture sufficiently long base fabrics for all press types. Further, it is possible to weave base fabrics provided with seam loops, and a necessary number of the base fabrics can be connected to form a closed loop. The seam loops at the ends of the base fabrics to be connected are intermeshed and engaged by inserting a seam yarn in a channel formed by the seam loops. The base fabric with seam loops can be woven by the known methods as flat weaving or horseshoe weaving. When woven as a plane, the weft yarns form seam loops on both edges of the fabric. Horseshoe weaving, in turn is carried out endless, so that only one edge of the fabric is provided with seam loops formed by weft yarns. Thus the base fabric can be assembled from a plurality of flat, or correspondingly, horseshoe portions, which are interconnected at their ends to form one longer entity. The press felts with seam loops have a drawback that at the seam the base fabric is different from other portions, whereby the properties of the felt at the seam area are different. In addition, the attachment of the batt fibre to the base fabric is difficult at the seam loop area, where yarn density is lower. During use, the felt is subjected to tensile stress, because of which the seam area, in particular, tends to deform. This weakens the adherence of the batt fibre even further and affects the permeability properties of the seam area.
The object of the present invention is to provide a novel and improved press felt for a paper machine, a base fabric and a method of manufacturing the press felt.
The method of the invention is characterized by arranging, in assembling the base fabric, prior to batt fibre attachment, transverse jointing edges of one or more base fabric modules edge on edge, substantially closely against one another, and connecting them at the resulting butt seam to form a closed loop.
Further, the press felt of the invention is characterized in that the base fabric modules to be connected are substantially as wide as the press felt and that, prior to attaching the batt fibre, the transverse jointing edges of the base fabric modules are arranged with a butt seam edge on edge, substantially closely against one another, to form a closed loop of a desired length.
Further still, the base fabric of the invention is characterized in that the base fabric modules to be connected are substantially as wide as the press felt and that the transverse jointing edges of one or more base fabric modules are arranged with a butt seam edge on edge, substantially closely against one another, and connected at the butt seam to form a closed loop of a desired length.
The basic idea of the invention is to form a base fabric having a laminated structure for the press felt, i.e. multi-layered base fabric having at least two superimposed, independent layers. Each of these independent layers comprises weft yarns and warp yarns of its own. According to the inventive idea, the base fabric is assembled of one or more base fabric modules to form a closed loop of a desired length. For assembly, at least one base fabric module edge, transverse to the paper machine direction, is provided with a jointing edge. On assembly, the jointing edges of one or more base fabric modules are interconnected with a butt seam. Each of the base fabric portions to be connected is substantially as wide as the press felt, i.e. seen in the lateral direction the press felt is continuous. Typically, one or more batt fibre layers are attached to the base fabric, preferably by needling.
Thanks to the butt seam according to the invention, the properties in the seam area of the laminated base fabric now substantially correspond to those in the rest of the base fabric. Thus, the base fabric can be formed more freely than before of a plurality of different base fabric modules. It is possible to form the base fabric modules by different solutions, as regards the shape, the yarn material, the dimensions and the thread count, the weave, the number of superimposed laminated layers, etc., whereby different base fabrics can be customized for different press sections.
In general, the laminated felts have an advantage that it is easier to provide the laminated felts with a sufficient space for water, but nevertheless an even surface. Moreover, the laminated felts remain longer unclogged. The distribution of pressure is more even in the laminated felts, and consequently they are less liable to permanent compression. In the laminated felt the compression of the batt fibre is no longer so critical a factor as previously Further, when the laminated felt is used, less vibration occurs in the press section, because variations in the felt thickness are smaller.
According to an embodiment of the invention, one or more base fabric modules are interconnected by overlapping the connecting portions provided in their jointing edges. For said lap seam the first and the second jointing edges of the base fabric modules to be connected are provided with a seam area, where the independent layers of the module form at least two connecting portions of different lengths. The connecting portions of different lengths are in inverted positions in the first and the second edges to be connected such that, on assembly, the connecting portions of the first edge and the connecting portions of the second edge fit edge on edge against one another to form a butt seam. Thus, the connecting portions of the first and the second jointing edges are in an alternating order in the seam area, and therefore the butt seams between the connecting portions are at different points in the seam area in the plane of the base fabric. Thanks to the lap seam, the butt seam between the base fabric modules becomes sufficiently strong, and hence it resists well the loading, to which the felt is subjected when the machine is run. The needled batt fibre binds the overlapping connecting portions.
According to an embodiment, one or more separate base fabric modules are interconnected by means of a lap seam at their cross machine direction ends. Thus is obtained one continuous closed loop, which is longer than individual base fabric modules. The solution expedites and facilitates the manufacture of press felts, because a base fabric preform can be woven in advance into a long mat, which will be cut to a desired size for a base fabric module. It is relatively easy and quick to provide the ends of the module with seam areas for assembly according to the invention.
The basic idea of an embodiment of the invention is that the base fabric of the press felt is woven in a plane form, whereby the weaving direction and the running direction of the fabric on the paper machine remain the same. Further, the base fabric comprises two or more superimposed, independent layers, which are woven simultaneously on the same weaving machine, each independent layer comprising its own yarn system such that the layers can be detached, if desired, from one another to form separate components. Each independent layer may have the same or different weave with respect to the other layers, likewise the yarn density, material, cross section and dimensions can be the same or different in each layer. During the weaving process of the base fabric the layers are bound together at desired intervals with a weft- or warp-oriented binding yarn system separate from the yarn systems of the layers. Alternatively, a necessary number of single weft or warp yarns of one or more independent layers may travel at predetermined intervals between two or more layers and thus bind the independent fabric layers together in the desired manner. The base fabric preform can be woven directly into a module of a predetermined length, or the base fabric preform can be woven into a continuous strip, wherefrom the base fabric modules of desired length are cut. A seam area of various layers of the base fabric is provided at both transverse ends of the base fabric module. For making a seam, the binding between the superimposed layers is released by cutting the yarns that bind the layers together and by removing said yarns from a predetermined portion at both ends of the base fabric. Thereafter the different layers are detached from one another for the length of the seam area. The detached superimposed layers are cut in a preplanned manner with respect to one another into at least two connecting portions of different lengths. The opposing module ends are cut to be mirror images of one another. An advantage with the above-described solution is that the base fabric can be woven flat, which allows the use of a simpler weaving machine which is also faster than the weaving machines suitable for flat weaving. Flat weaving also provides a better possibility to customize the structure of the base fabric, because it is not connected with the shuttle rotation of the weaving machine. Moreover, because the weaving direction and the running direction of the flat woven base fabric are the same, it is possible to utilize the advantages of the warp-dominating fabric. The warp yarns can be flattened, their number can be great, and the yarn interlacing can be selected such that the surface of the base fabric becomes even.
The basic idea of an embodiment of the invention is that the base fabric of the press felt comprises at least one first base fabric module ready-woven into a closed loop and at least one second base fabric of a planar shape. The first and the second base fabric modules comprise one or more layers and their weave and yarns are suitably selected. The first and the second base fabric modules are superimposed, and the transverse jointing edges of the second, planar base fabric module are interlinked with a butt seam. There can be a plurality of base fabric modules woven into a loop as well as flat-woven modules, and further, they can be arranged one on top of the other in a desired manner. An advantage with this base fabric structure is that the loop-woven base fabric module receives the machine direction loading, to which the base fabric is subjected. The flat woven base fabric modules, in turn, contribute to the properties of the base fabric surface, as well as to the water volume, density, etc. In addition, the planar module is easy to manufacture by flat weaving.
It should be noted that in the present application the butt seam refers to a transverse base fabric seam, in which two layers in the same plane are arranged edge on edge, and the jointing edges, transverse to the plane surface of the base fabric, are disposed substantially closely against one another without any seam loops or like securing means, whereby the seam area substantially corresponds to the structure in the rest of the base fabric.